How Does a Lateral Flow Test Work?

 by Yuanzheng Mao 

Since the outbreak of COVID-19, up to when this article was written, almost 500 million virus tests have been reported in solely England. [1] With so many tests done, people are able to keep an eye on themselves and avoid the spread of virus to some extent. During this period, rapid lateral flow (LFD) tests have definitely been playing a key role. This technology is like magic. Because once you’ve dropped the sample, in only about 10 min, it will automatically tell you whether you have a traceable amount of virus with one or two red lines. What’s more, you don’t have to know a lot of science to carry out the test. Almost everyone can do it, provided one has got proper instructions. So, being so efficient and simple, how does a lateral flow test work? [2] [3] [4] [5]

First, let’s look at the structure of a later flow test device. (Figure 1) A strip is composed of a sample pad, a conjugate pad, a test line, a control line, and an absorption pad.

 

Figure 1 Structure of a lateral flow test strip. Picture from https://www.lateralflows.com/lateral-flow-assays/

A sample pad is where one drops the sample onto. From there, the sample moves along the strip by itself and reaches the conjugate pad. On the conjugate pad, there are some molecules (A) that can only combine with the target molecules (T) in the sample, which are labelled with other visible particles (B).

The mixture continues moving. Then, it reaches the test line. There are some molecules (C) fixed here. Like A, they also bind specifically to T. So, if, in the sample, there are T present, they will be immobilised by C. Since T have been attached to A and A are labelled with B, people can recognise a clear test line, due to the gathering of many B.

As the mixture move forwards, it arrives at the control line. Here, there are another type of molecules that can only attach to and immobilise A. One can tell if the device works by checking the control line. If it does, extra unbound A can be fixed here. Again, like the test line, due to the gathering of many B, one can clearly see the control line. If it doesn’t (e.g., the molecules on the conjugate pad are not present or they aren’t effective for some reason), then there will be no control line there.

The absorption pad is for collecting the leftover.

Figure 2 An illustration of how a lateral flow device works. Picture from https://en.wikipedia.org/wiki/Lateral_flow_test#/media/File:Lateral_Flow_Assay.jpg

There are many applications of a lateral flow test, such as a home pregnancy test [2] and a COVID-19 test. Take a COVID-19 lateral flow as an example. On each virus, there are approximately 20 spike proteins. If the sample contains the virus, once it reaches the conjugate pad, the immobilised antigens labelled with gold nanoparticles (the gold nanoparticles look red) on the conjugate pad will bind to the spike proteins on the virus. Then, as the mixture travels further, the surface-immobilised antibodies on the test line bind to the vacant spike proteins. Therefore, a red test line appears. Further on, some extra gold nanoparticle-labelled antigens become immobilised by the molecules on the control line, making it red which indicates that the device works. [6]

References

[1] GOV.UK (2022), Testing in England [online] Last accessed 20 June 2022: https://coronavirus.data.gov.uk/details/testing?areaType=nation&areaName=England

[2] Wikipedia (2022), Lateral flow test [online] Last accessed 20 June 2022: https://en.wikipedia.org/wiki/Lateral_flow_test

[3] DCN Dx (2018), Lateral Flow Immunoassays: How Does a Lateral Flow Device Work? [online] Last accessed 20 June 2022: https://dcndx.com/lateral-flow-rapid-diagnostic-test/

[4] Lateral Flows (2022), Lateral flow assays [online] Last accessed 20 June 2022: https://www.lateralflows.com/lateral-flow-assays/

[5] Chemistry at Hull (2021), Lateral Flow Test Teardown [online] Last accessed 20 June 2022: https://www.youtube.com/watch?v=AERyetmR7FY

[6] Royal Society of Chemistry (2022), 54^th International Chemistry Olympiad 2022 UK Round One Student Question Booklet [online] Last accessed 20 June 2022: https://edu.rsc.org/download?ac=518812